The present invention relates to the field of a semiconductor processing and more specifically to a method and apparatus for controlling the deposition of a silicon film.
FIG. 1 illustrates an example of a radiantly-heated semiconductor substrate processing chamber. Such chambers are generally used at process pressures less than or approaching 100 Torr. The single substrate reactor 100 includes top wall 132, sidewalls 133 and bottom wall 134 that define the reactor 100 into which a single substrate, such as a wafer 102, can be loaded. The wafer 102 is placed on susceptor 105 that is rotated by motor 137 to provide a time averaged environment for the wafer 102 that is generally disk-shaped. The susceptor and wafer are heated, and process gases are pumped through the chamber 130. The process gases flow across the surface of the wafer in the direction of arrows 141. The process gases contain the chemical species that react at the heated wafer surface to form a film on the wafer. The wafer is rotated in an effort to provide uniform gas depletion across the wafer.
Preheat ring 140 is supported in the chamber 130 and surrounds the wafer 102. The wafer 102, susceptor 105, and preheat ring 140 are heated by light from a plurality of high intensity lamps 138 and 139 mounted outside of reactor 100. Top wall 132 and bottom wall 134 of chamber 130 are typically made of quartz and are substantially transparent to light to enable the light from external lamps 138 and 139 to enter reactor 100 and heat susceptor 105, the wafer 102, and preheat ring 140.
Although the rotation of the substrate and thermal gradients caused by the heat from lamps 138 and 139 can affect the flow profile of the gases in reactor 100, the dominant shape of the flow profile is a laminar flow from the gas input port 110 and across preheat ring 140 and the wafer to exhaust port 111.
In a radiantly-heated reactor 100, the temperature within the chamber is measured optically with a pyrometer 150 that is typically located below the chamber 130. The pyrometer 150 measures the optical intensity 152 emitted by the heated susceptor 105. Since the radiation emitted by the heated susceptor depends on the susceptor temperature, the susceptor temperature can be calculated by measuring the intensity with the pyrometer 150. Because the emissivity of the susceptor is dependent on the surface conditions of the susceptor and the quartz dome or bottom wall 134 through which the emissivity of the susceptor is measured, the wafer temperature is not directly measured and therefore can be inaccurate. A pyrometer 150 is typically used to measure the susceptor temperature, or to determine the wafer temperature, because it is difficult to physically measure the temperature of the wafer during processing because the wafer rests on the rotating susceptor 105. Because the susceptor is a rotating body, it is difficult to attach a measuring device such as a thermocouple directly to the susceptor to physically measure the temperature of the susceptor. Also, since the emissivity of the heated susceptor is measured by the pyrometer 150 through the quartz wall 134, and is dependent on the surface conditions of susceptor 105 and the quartz wall 134, it is necessary to periodically clean the surfaces of the chamber including the quartz wall 134 and the bottom surface of the susceptor 105, because the residue from the processing gases tends to accumulate on these surfaces and can affect the emissivity of the surfaces, thus introducing inaccuracy in the temperature measurement.
The uniformity of film thickness is measured in two ways. First, wafer-to-wafer uniformity is measured, and also uniformity across the surface of individual wafers is measured.
Since the film thickness is dependent on temperature, among other parameters, it is important to accurately control the temperature within the processing chamber. Therefore, the thermal deposition processes that are performed in such a chamber having optical temperature measurement can be limited by the relative inaccuracy of such a temperature measuring system.
Current film deposition reactors such as reactor 100 shown in FIG. 1 use hydrogen as a carrier or dilution gas. Hydrogen is used because hydrogen gas has a relatively high thermal conductivity (as compared to nitrogen, for instance). The thermal conductivity of hydrogen gas provides a large enough temperature gradient between the wafer and the chamber or reactor dome. A relatively large temperature gradient helps to avoid gas phase nucleation which results in silane decomposition on the dome and a resulting coating on the dome. When gas phase nucleation and dome coating occurs, less of the gas species is reacted at the wafer, resulting in non-uniform film thickness on the wafer. Hydrogen gas and a large temperature gradient can reduce silane decomposition due to gas phase nucleation and dome coating. Consequently, less of the gas species is used, resulting in a less efficient process.
A radiantly heated film deposition chamber therefore is very sensitive to process fluctuations, and in particular, temperature fluctuations which result in potentially non-uniform film thickness. One problem associated with fluctuating wafer temperatures is non-uniform film thickness of the wafer. Significant effort has been expended to improve process parameters to increase uniformity of film thickness, both on a wafer-to-wafer and individual wafer basis. There are also problems associated with the rotation of the susceptor, such as wobble or vibration, which require highly complex solutions.
In radiantly-heated processing reactors, the feed stock consumption is relatively high, meaning that the amount of reactant such as silane or disilane used compared to the amount of product deposited (i.e., deposition rate of the film) is high resulting in a low process efficiency. Also, because a large amount of feed stock is used, these types of reactors require frequent maintenance, thus increasing the cost and down time of the processing machinery.
Another semiconductor substrate process in which thickness uniformity and repeatability is important is chemical vapor deposition (CVD). CVD amorphous silicon films have been used in gap fill applications due to the excellent step coverage ability. With the shrinkage of device geometry, it is desirable that the deposited film has a conformal gap fill profile for the sub-micron patterns. Most amorphous silicon films are currently batch processed by furnaces, even though furnaces have the disadvantage of long cycle time. Furthermore, the low temperature nature of the amorphous silicon process limits the throughput during manufacturing. A process which could achieve excellent gap filling quality and high throughput is desirable in single wafer deposition technology.
A method for depositing doped polycrystalline or amorphous silicon film is described. The method includes placing a substrate onto a susceptor. The susceptor includes a body having a resistive heater therein and a thermocouple in physical contact with the resistive heater. The susceptor is located in the process chamber such that the process chamber has a top portion above the susceptor and a bottom portion below the susceptor. The method further includes heating the susceptor. The method further includes providing a process gas mix into the process chamber through a shower head located on the susceptor. The process gas mix includes a silicon source gas and a carrier gas. The carrier gas includes nitrogen with hydrogen as an additional dilution gas. The method further includes forming the doped silicon film from the silicon source gas.